1. Field of the Disclosure
The present invention relates to an ophthalmological device for processing eye tissue by means of femtosecond laser pulses. The present invention relates, in particular, to an ophthalmological device including a projection optical unit for the focused projection of the femtosecond laser pulses into the eye tissue.
2. Related Art
For processing eye tissue by means of laser beams, an image region or a working region is scanned with laser pulses by means of the pulsed laser beam being deflected in one or two scanning directions by means of suitable scanner systems (deflection devices). The deflection of the light beams or of the laser pulses, for example femtosecond laser pulses, is generally performed by means of movable mirrors which are pivotable about one or two scanning axes, for example by means of galvanoscanners, piezoscanners or polygon scanners.
U.S. Pat. No. 7,621,637 describes a device for processing eye tissue, said device having a base station with a laser source for generating laser pulses and a scanner arranged in the base station with movable deflection mirrors for deflecting the laser pulses in a scanning direction. The deflected laser pulses are transmitted via an optical transmission system from the base station to an application head, which moves over a working region in accordance with a scanning pattern by means of a mechanically moved light projector. The deflection in the scanning direction, which is much faster compared with the mechanical movement, is superimposed in the application head onto the mechanical movement of the light projection and thus onto the scanning pattern thereof. A fast scanner system in the base station enables a fine movement of the laser pulses (microscan), which is superimposed onto the scanning pattern of the movable light projector that covers a large working region, for example the entire eye.
With the availability of faster laser pulses that yield ever higher pulse rates, for example more than one million pulses per second (MHz), the known scanner systems encounter their physical limits of being able to position pulses separately, and the pulse rate of the lasers has to be artificially reduced. In particular, the mechanical movement of light projectors or lenses for scanning working regions, and also the mass inertia of galvanometer scanner systems, which does not permit arbitrarily high accelerations, limit the possible scanning patterns and scanning trajectories to the effect that greater changes in direction have to be avoided, and that the pulse rate has to be actively reduced (in a complicated manner) or the laser has to be switched off when the minimum scanning speed is undershot, for example at reversal points. Consequently, the known scanner systems impose significant limits on cut guidances that can be implemented. From a clinical standpoint, however, it is desired to plan the cut profile according to the biomechanical behaviour of the tissue, and not necessarily according to the speed and bandwidth of the scanner system, as is carried out with the known scanner systems. In contrast to surface processing, when cutting soft tissue, for example eye tissue, it is not always possible to employ simple pulse scanning patterns, for example line or spiral patterns, since tissue deformations can be caused by internal evolution of gas or release of stresses, and it is necessary to avoid said tissue deformations by means of suitably more complex scanning patterns taking account of the expected biomechanical behaviour of the tissue. Although the known scanner systems make it possible to process simple scanning patterns, for example to cut a tissue flap, this generally being performed as a large area segment with a simple edge geometry, in the case of isolated processing regions with a complicated edge geometry, such as are required for refractive correction, for example, or in the case of other, biomechanically governed more complex scanning patterns, that is no longer possible in such a simple manner. By way of example, it is then necessary for a large-area scanning pattern to be covered with a mask (electronically or optically), or the small regions are processed, e.g. scanned, individually, which results in a corresponding reduction of the processing speed, since the scanner system has to decelerate and accelerate very frequently relative to the section to be scanned.